If you are referring to the web-browser's previous and back buttons you may have a few problems with this.
When you hit the "back button" (or previous button) a cached version of the page is displayed to you. If your page doesn't use Ajax to post information back to the server it is quite usual for these pages to retain everything selected by the user during the last full page submit to the server. (Same goes for hitting the "next" button too).
Sometimes, if you have managed to clear the user's cache (or specified that the page shouldn't be cached), or if your page uses Ajax to submit information to the server, the page will be displayed as it was originally.
If you aren't referring to the web-browsers back/next buttons...and you have links or buttons as part of your page to help with user navigation....then you can store the information in Session so that it is available to all pages.
Please explain, in more detail, what you mean by "next/previous" buttons....and explain try to explain how/why you think the information is being lost.
-Frinny